This invention relates to an image or picture reader and, particularly, to an image reader which measures or photometers, on scanning the whole image or picture area, the quantity of transmitted light, reflected light or luminescence light given from individual picture elements comprising the picture and outputs a time-serial electric signal reflecting the measured results, for the purpose of preserving, processing or forwarding the image information.
An image reader reads the image information of an object picture by detecting the variation in quantity of transmitted, reflected or luminescene light given from individual picture elements. However, this light quantity varies over a wide dynamic range in general. For example, the dynamic range of transmitted light from an X-ray film extends over a fixed quantity of incident light, four digits of magnitude in the case of diffuse density photometry, or six to eight digits of magnitude in the case of specular density photometry. Also, the quantity of light emitted by a fluorescent screen used in X-ray fluoroscopy gives a dynamic range of six or more digits.
Contrary to the above, the dynamic range of detection systems employed in conventional image readers is restricted by the dynamic range of detectors and amplifiers making up the detection system and gives four digits of magnitude at the most. For example, a CCD (charge coupled device) has a two-digit dynamic range, a camera tube gives three digits or so, and an amplifier has a dynamic range only in the order of three to four digits where all of these components serve as the detector.
Therefore, since a conventional image reader has a detection system whose dynamic range is less than that of the light quantity from the object picture, it cannot fully read the image information of this picture.